This invention relates to a fuel gas steam reformer assembly. More particularly, this invention relates to an autothermal fuel gas steam reformer assembly which employs an open cell foam catalyst bed that reduces the size and weight of the reformer, assembly.
Fuel cell power plants include fuel gas steam reformers which are operable to catalytically convert a fuel gas, such as natural gas or heavier hydrocarbons, into the primary constituents of hydrogen and carbon dioxide. The conversion involves passing a mixture of the fuel gas and steam through a catalytic bed which is heated to a reforming temperature which varies depending upon the fuel being reformed. Catalysts typically used are nickel catalysts which are deposited on alumina pellets. There are three types of reformers most commonly used for providing a hydrogen-rich gas stream to fuel cell power plants. These are a catalytic steam reformer, an autothermal reformer, and a catalyzed wall reformer. In addition, hydrocarbon fuels a may be converted a hydrogen-rich gas stream by use of a partial oxidation reaction apparatus. A typical catalytic steam reformer will consist of a plurality of reaction tubes which are contained in a housing that is insulated for heat retention. The reaction tubes are heated by burning excess fuel gas in the housing and passing the burner gas over the reaction tubes. The reforming temperature is in the range of about 700xc2x0 F. to about 1,600xc2x0 F. The individual reaction tubes will typically include a central exhaust passage surrounded by an annular entry passage. The entry passage is filled with the catalyzed alumina pellets, and a fuel gas-steam manifold is operable to deliver the fuel gas-steam mixture to the bottom of each of the entry passages whereupon the fuel gas-steam mixture flows through the catalyst beds. The resultant heated mixture of mostly hydrogen and carbon dioxide gas then flows through the central exhaust passages in each tube so as to assist in heating the inner portions of each of the annular catalyst beds; and thence from the reformer for further processing and utilization. Such catalytic steam reformers are described in U.S. Pat. No. 4,098,587.
A typical autothermal reformer may-be a single bed or a multiple bed tubular assembly. Autothermal reformers are often used when higher operation temperatures are required for the reforming process because the fuel to be processed is more difficult to reform. In an autothermal reformer, the reaction gasses are heated by burning excess fuel within the reaction bed by adding air to the fuel and steam mixture so that the remaining fuel-steam mixture is increased to the temperature necessary for the fuel processing reaction. Typically, wall temperatures in an autothermal reformer are in the range of about 1,400xc2x0 F. to about 1,800xc2x0 F. Such reformers are described in U.S. Pat. No. 4,473,543.
A third type of prior art reformers have utilized catalyzed wall passages such as described in U.S. Pat. No. 5,733,347. Such reformers are formed from a sandwich of essentially flat plates with intervening corrugated plates which form reformer gas passages and adjacent regenerator-heat exchanger passages. Each of the reformer passage plate units is disposed directly adjacent to a burner passage plate unit so that the adjacent reformer and burner passages share a common wall.
Besides the reformer devices described above, a partial oxidation reaction apparatus may also be used to produce a hydrogen-rich fuel stream. This device is typically a chamber that is led a hydrocarbon fuel, steam and oxidant source, usually air, so that the mixture spontaneously partially oxidizes to form a hydrogen-rich mixture. Such devices, for example, are disclosed in PCT application WO 98/08771.
U.S. Pat. No. 4,451,578, granted May 29,1984 contains a discussion of autothermal reforming assemblages, and is incorporated herein in its entirety. The autothermal reformer assembly described in the ""578 patent utilizes catalyzed alumina pellets. Although autothermal reformers allow a degree of system compaction, it would be desirable to further decrease the size and weight of an autothermal reformer, and also of any tubular reformer, particularly in systems which are utilized in vehicular applications. Attempts have been made to decrease the size and weight of autothermal and other tubular, reformers through the use of specially configured catalyst pellets. Such specialized pellet configurations include rings, flat pellets with holes, wagon wheel-shaped pellets, and lobed pellets, for example.
It would be desirable to provide an autothermal reformer assembly which does not require the use of specially configured catalyzed alumina pellets, and which is more compact and light weight than the prior art autothermal reformer assemblies which do utilize catalyzed alumina pellets. Such reformer assemblies would find particular utility in vehicular applications.
This invention relates to a fuel cell system autothermal reformer assembly which provides an enhanced catalyst and heat transfer surface area; is compact and light weight; and provides an enhanced gas mixing and distribution flow path. The catalyst bed structure of this invention is formed from a monolithic open cell foam core which is provided with a porous high surface area wash coat layer onto which the catalyst layer is deposited. Cells in the open cell foam onto which a catalyst is deposited are occasionally referred to herein as xe2x80x9ccatalyzed cellsxe2x80x9d. The wash coat may be alumina, lanthanum-stabilized alumina, silica-alumina, silica, ceria, silicon carbide, or another high surface ceramic material. The choice of wash coat will depend on the operating parameters of the specific catalyst bed.
The monolithic gas flow component is a foam with interconnected open cells, the surfaces of which are catalyzed with a catalyst. The foam monolith has an entry end portion which is coated with a catalyst consisting of lanthanum-promoted alumina, calcium oxide, and an iron oxide catalyst which can also be treated with a small amount of platinum, palladium or rhodium for improved low temperature fuel gas ignition. As an alternative configuration, the entry end may include a catalyst of platinum, palladium or rhodium without the iron oxide catalyst. The remainder of the foam monolith is provided with a nickel, copper or zinc catalyst, or with such noble metal catalysts such as platinum, palladium, rhodium, or the like. The open cell foam, once wash coated, provides the high surface area base required in order to achieve the deposition of the high surface area catalysts needed to properly process the fuel gas. The open cell foam also provides an enhanced mixing and distribution gas flow pattern for gases passing through the monolith since the gases will flow both laterally and longitudinally through the structure. The open cell foam also provides high surface area heat transfer paths that contribute to a more turbulent gas flow that enhances heat transfer rates in systems utilizing the catalyst bed. Additionally, the high heat transfer provided by the foam can be continued into and through adjacent walls of the reactor so as to create a highly efficient heat transfer device that results in improved process temperature control and reduces the size and weight of the reformer for a given output level. The intervening walls may be flat plates or they may be cylindrical walls with heat transfer capabilities. The monolithic open cell foam catalyst bed may be bonded to the reformer catalyst bed walls by brazing, or any other appropriate mechanism which is suitable for the system in question. When a ceramic foam catalyst bed is employed, the catalyst bed will not likely be bonded to the reformer bed walls.
All surfaces to be catalyzed will be primed by means of a conventional wash coating process such as that provided by United Catalyst, W. R. Grace and Co., or Englehard Corp. The wash coating process produces a porous layer on all surfaces of the foam, which layer forms a base for the catalyst coating. It will be understood that the interstices as well as the outside surfaces of the open cell foam monolith are wash coated and are also catalyzed. Since the catalyst beds are of minimal size and weight, they are especially suited to vehicular applications where size and weight are critically important, and because vehicle applications require rapid start-up capability that is closely dependent on the size and weight of the components. Small, light weight catalyst and reactant beds can be rapidly heated with a minimum energy input.
In its preferred embodiment, the autothermal reformer assemblage is generally cylindrical in configuration, and includes several concentric chambers formed from cylindrical housing walls. The outermost chamber is annular and contains helical fuel/steam, and/or air flow conduits. In an alternate configuration, the outermost conduit could contain a steam/air mixture with the fuel being injected into the steam/air mixture just prior to entry into the entry portion of the catalyst bed. A central gas flow chamber is cylindrical and contains a monolithic open cell foam catalyst bed through which the fuel, steam and air gas streams flow. The catalyst bed has two different catalyzed zones, one of which is an inlet zone for carbon-free combustion of a small amount of the fuel gas and for autothermal reforming of the fuel gas, and the other of which is an adiabatic reforming zone. An intermediate annular gas flow chamber is interposed between the inner gas flow chamber and the outermost chamber and serves to channel the reformed gas stream which issues from the central chamber to the outermost chamber.
Heat is generated internally in the reformer catalyst bed due to the addition of air to the process gas stream, which is a fuel/steam mixture. Thus there is no need to spread the reforming catalyst along large heat transfer surfaces as is required with a conventional steam reformer. The use of an open cell foam monolith as the catalyst support bed can increase the catalyzed surface area of the support bed by a factor of at least three, depending on the pore size of the foam catalyst support bed. Use of the foam support bed can also reduce the volume of the bed by a factor of at least three, and can reduce the weight of the bed by an even greater factor, as compared to a pelletized catalyst bed. The foam catalyst support bed can provide greater than an 80% open volume while a pelletized bed provides about a 40% open volume. The increased open volume results in a much lower pressure drop across the catalyst bed than can be achieved with a pelletized bed.
Start-up of the reformer can be achieved by either pre-heating the bed with a hot gas, such as steam, or by fabricating either the entire monolith or just the inlet section with a conductive resistance monolith element. The resistance element can be connected to an electrical source such as a car battery which will enable the monolith to reach operating temperatures within less than twenty seconds.
An example of the benefits of the foamed catalyst support is that, for an equivalent catalyzed surface area, a pelletized autothermal catalyst bed has a volume of 0.25 ft3, weighs about twenty five pounds; while a foam support bed having a twenty pore/finch pore size can be formed with a volume of 0.07 ft3, and weighs less than four pounds. In cases wherein a higher pressure drop can be tolerated, a 30 to 40 pore/finch foam can be used to provide an even smaller and lighter bed. Such small and light weight to fuel processors can be heated quickly using electricity from an automobile battery, and thus are particularly desirable for vehicular applications.
In a preferred embodiment of the invention, the catalyst bed will be formed from a ceramic foam that will be wash coated with alumina, calcium aluminate or lanthanum-promoted alumina, whereafter the wash coat will be impregnated with a proper catalyst. The inlet section of the catalyst bed will be provided with an iron oxide catalyst which limits carbon deposition and is, at the same time operable to raise the temperature of the incoming gas stream without requiring large amounts of oxygen as described in U.S. Pat. No. 4,451,578. The iron catalyst may be promoted with a small quantity of platinum, palladium or rhodium so as to provide a lower ignition temperature of the fuel gas fraction that is combusted in the reformer during operation of the reformer. In the case of an autothermal reformer with a conventional catalyst that includes iron oxide as described in U.S. Pat. No. 4,451,578, the expected ignition temperature of the fuel is in the 1,000 to 1,300xc2x0 F. range. However, it has been found that with the addition of a small amount of a noble metal catalyst such as platinum, palladium or rhodium to the iron oxide, the ignition temperature can be lowered to about 500xc2x0 F. The metal catalyst addition is in the range of about 0.01% up to about 1.0% by weight. Typically, the amount added is about 0.1%. The achievement of a lower ignition temperature enables a quicker start-up of the reformer which is particularly desirable in vehicular applications of the system. The iron oxide in the catalyst could be replaced by platinum, palladium or rhodium as described in U.S. Pat. No. 4,415,484.
The inlet portion of the catalyst bed is operable to bum oxygen and a minor amount of the fuel gas so as to raise the temperature of the inlet portion of the catalyst bed. This raises the temperature of the gas stream to temperatures which provide enhanced conversion of the fuel stream to a high hydrogen-content gas. Minimal oxygen requirements serve to inhibit carbon formation in the catalyst bed. Other carbon formation-suppressing catalyst bed components such as calcium oxide, lanthanum oxide, and/or cerium oxide, for example, could also be used. The subsequent reforming section of the catalyst bed is provided with a conventional nickel or noble metal reforming catalyst, or a combination of a nickel reforming catalyst, and a noble metal reforming catalyst.
It is therefore an object of this invention to provide an improved autothermal fuel gas processing assembly which includes a catalyst bed that is compact and light weight.
It is a further object of this invention to provide a fuel gas processing assembly of the character described which is sufficiently compact and light weight so as to be useful in a vehicular application.
It is another object of this invention to provide a fuel gas processing assembly of the character described which can be quickly brought up to operating temperatures through the use of electricity provided by an automotive battery.
It is yet another object of this invention to provide a fuel gas processing assembly of the character described which has a lowered fuel ignition temperature to allow rapid start up.